1. Field of the Invention
The present invention relates to a method of determining the travel time of an elastic signal from a predetermined point of a drill string to a transducer spaced on the drill string from the predetermined point.
2. Description of the Related Art
During the drilling of a well, it is useful to know the time taken by an elastic wave to travel from one part of the drill string to another, for instance from the bit to a transducer placed near the top of the drill string. Although an estimation of the travel time may be made on the basis of the length of the drill string and the speed of propagation of the energy in the material of which the drill string is constructed, such an estimation does not correlate with actual travel times. This is because the drill string is made up of a plurality of sections with impedance discontinuities between the sections. The impedance discontinuities create energy reflections which interact with each other and with the direct upwardly propagating energy to alter the effective velocity of propagation of the energy within the drill string.
Knowledge of propagation time along the drill string is particularly useful for the technique known as "checkshot while drilling". According to this technique, the energy generated by the drill bit and propagating into the earth is used as a seismic source. Direct and reflected energy is detected by a transducer such as a geophone, for instance located on the surface of the earth some distance away from the drill string. A further transducer is mounted near the top of the drill string. The outputs from the transducers may then be processed to reveal information, for instance about the strata through which the drill bit is passing or ahead of the drill bit. However, in order to make use of the transducer output signals, it is required to know the time taken by the acoustic energy generated at the drill bit to travel up the drill string to arrive at the transducer on the drill string.
The applicants are presently aware of the following prior art, none of which is considered relevant to the present invention: EP-A-0409304; U.S. Pat. No. 4,926,391; U.S. Pat. No. 4,718,048; U.S. Pat. No. 4,829,489; and "The Use of Drill-Bit Energy as a Down Hole Seismic Source" by J. W. Rector III and B. P. Marion, Geophysics, Vol 56 No 5 (May 1991), pp 628-634.